294 DOCS. 306, 307 MARCH 1917 people existed, if I did not see them before me. Now I can only hope for salvation from an external force. Affectionate greetings also to Anna and Vero, yours, Albert. 307. From Friedrich Adler Vienna VIII, 1 Alser St., 9 March 1917 Dear Professor Einstein, Now that I have plenty of free time at my disposal, I have taken up my studies on the foundations of physics again, which I had abandoned 7 years ago.[1] I intended to assemble in a book my previously published and unpublished papers on Mach and to expand on them in various directions. The work had already progressed quite far when I started the chapter on relativity. Then something quite unexpected happened to me. Ever since taking Mach’s point of view (1903) I also assumed as a matter of course the relativity of rotation and already advocated it in 1907 in my lecture courses in Zurich.[2] You may still remember as well that around 1909 we had a longer argument about it in our attic rooms on Mousson Street.[3] It remained vividly in my memory because it was very important to me. I was very irritated, you see, that in taking the centrifugal effects into account, you rejected relativity for rotations. I asked myself at the time whether an error existed in Mach’s or my argumentation after all. But I could find none. So much the greater was my joy when your general theory of relativity came out. I did not have the time to follow the subject closely, however. It is only now that I sent for the more recent literature in order to be able to present Mach’s position on relativity. And there I saw, first in Freundlich’s brochure,[4] and then in your own papers,[5] that you have accepted Mach’s position entirely, including the centrifugal phenomena.[6] I was in ecstasy when suddenly, 4 weeks ago, a turning point came in my considerations which reveals the whole problem differently from how I had seen it previously. I found, first in a more recent discussion of Foucault’s pendulum experiment, and then generally, a criterion that you and Mach do not take into consideration, or at least, not sufficiently, which sheds new light on it all. I believe I have found where the error in the assumptions not only by Mach but also by you lies. I cannot discuss this in detail within the bounds of a letter, but just want to say that, naturally, it does not involve a return to “absolutes” but a criterion of a relativistic nature for preferred reference systems.[7] In the last 7 years, I was able to follow the course of the literature only very cursorily, of course, and now can catch up only with the most important papers.